1. Field of the Invention
The present invention relates to an image forming apparatus, a method of processing an image, and a computer-readable recording medium thereof. The invention more particularly relates to an image forming apparatus with a joint head in which a plurality of heads overlap each other at their end portions, each head includes a plurality of nozzles arranged in a predetermined direction, and the plurality of heads are arranged in a direction in which the nozzles are arranged, and relates to a method of processing an image.
2. Description of the Related Art
In an inkjet recording system, by using a recording head having ink tanks and nozzles which communicate with the ink tanks, and applying pressure to ink in the ink tanks in accordance with image information, ink droplets are allowed to fly from the nozzles and thus to be attached to a recording medium such as a paper sheet or a film to thereby form an image. Since an image forming apparatus (inkjet printer) employing the inkjet recording system forms an image in a non-contacting manner by discharging ink from the recording (print) head, it has a feature of being able to record an image on various types of recording media.
Ink jet printers are largely categorized into line type (line printer) and serial type (serial printer).
The line printer is a printer that forms an image by arranging a recording head in a fixed manner in which nozzles are arranged, the nozzles are arranged in almost the entire range of the width of a sheet. This type of printer conveys a paper sheet at high speed and forms an image over the entire area of the sheet width by single scanning, and accordingly, the productivity is very high. On the other hand, since the type of printer needs to form an image by single scanning, there is a disadvantage that an ejection defect or the like of the head is directly reflected on the image quality. In addition, since the mounting of a head having a length extending over a sheet width causes many problems such as a low yield and the like, in practical applications, the length of the sheet width is generally covered by using a joint head acquired by aligning, in the main-scanning direction, a plurality of heads each of which having a plurality of nozzles arranged in the main-scanning direction (sheet width direction), and accordingly, there are problems in the image such as color unevenness or a stripe that is caused by a difference in the features of the heads or an assembly error.
On the other hand, the serial printer is a type of printers that forms an image by reciprocating a recording head in a direction (main-scanning direction) perpendicular to the sheet conveying direction (sub-scanning direction). Since the serial printer forms an image by performing scanning a plurality of times, the resolution can be increased in a relatively easy manner, and low cost and the miniaturization thereof can be easily realized, therefore serial printers are widely used. However, since the formation of an image for printing one paper sheet is performed through scanning a plurality of times, the productivity of the serial printer is lower than that of the line printer. Thus, recently, in order to improve the productivity of the serial printer, it is a known fact that a higher speed is achieved by increasing width of the head in the sub-scanning direction thereby increasing the printable width (a length in the sub-scanning direction) through single scanning, or by using a joint head acquired by aligning, in the sub-scanning direction, a plurality of heads each of which having a plurality of nozzles arranged in the sub-scanning direction (sheet conveying direction). In such a case, even in a serial printer, similarly to the case of a line printer, there are problems that are inherent in a joint head such as a difference in the features of the plurality of heads and misalignment in the assembly.
Hereinafter, the problems in the joint head will be described in detail.
In the joint head, a plurality of heads are connected together, and accordingly, a fluctuation in the size, the shape, the landing position, or the like of discharged dots may occur due to a manufacturing fluctuation of the head or the driving system, whereby there is a possibility that density unevenness from head to head occurs. In order to solve this problem, there is a technique (Japanese Patent Application Laid-open No. 2009-234115) in which the input/output characteristics are corrected by performing the correction of the input/output characteristics of the individual heads. As an example of the correction, there is a method in which input/output correction is performed in γ correction or the like. This is, for example, a target density is approached by decreasing the output tone (gray scale, gradation) of a thick head from a reference to decrease the density, and increasing the output tone of a thin head from the reference to increase the density.
For example, as illustrated in FIG. 15, in a case where a head α has large discharge dots and a head β has small discharge dots for the same input data, there is a difference in the density (shading) like a dot pattern D1, and a color difference between the heads is created. Thus, the dot arrangement of the head α is changed to be sparser, and the dot arrangement of the head β is changed to be denser, whereby the densities are approached. This may be performed by using one head as a reference and correcting the other head or by correcting both heads to have the same target density. The latter case in which the characteristics of each head are adjusted to ideal image characteristics is more preferable. A dot pattern D2 is formed by approaching the characteristics of the head β to those of the head α.
At this time, when described in more detail, as can be understood from the example of the dot pattern D2, the number and the type of dots arranged on a sheet surface are changed. In a monitor such as a cathode ray tube (CRT) or a liquid crystal, although tones of multiple levels, for example 256 tones, can be represented by using one pixel by adjusting the luminance, the number of colors of ink that can be mounted in an inkjet printer is only about one to eight, and there is limitation of about one to three types such as black, dark gray, light gray, and the like on similar colors, and accordingly, it is difficult to represent the tones by changing the density (size) of each dot alone. Accordingly, the density (shading) is represented based on the amount of ink attached per unit area of a recording medium. By controlling the number of dots in a binary printer that can discharge dots of only one size and by controlling the number and the size of the dots in a printer in which the number of dots and a plurality of sizes (for example, four values of a large droplet, a medium droplet, a small droplet, and no droplet) can be handled, the amount of attachment of ink per unit area is controlled so as to represent a tone. Accordingly, γ correction in the inkjet printer is to control the arrangement of dots (the number and the size of dots) per unit area (for example, the number of thrown dots is decreased in a case where the dot is larger than a target dot, and the number of thrown dots is increased in a case where the dot is smaller than a target dot).
Accordingly, in a case where the γ correction (tone correction) of the input/output characteristics is performed as in the dot pattern D2, although the density is the same in a macroscopic view, there is a slight difference in the arrangement of dots, and that may cause that the conversion of the texture is visually noticeable. In addition, as for colors, even when the density is uniform, there may be a case where the hue differs. For example, when comparing a case where a small number of large dots are arranged and a case where a large number of small dots are arranged, even when the densities are at the same level, there may be a case where a change in the hue or saturation occurs due to the penetration characteristics of ink so as to have different impressions of the color. In addition, in the case of a color that is represented by overlapping a plurality of colors such as a multi-order color, since the tinge is adjusted by using a method of arranging dots, therefore, even when the tinge is adjusted in each color, there may be a case where the way colors are overlapped is changed so that the way of exposition of the colors can not be controlled, or there may be a case where, due to the difference in the way of penetrating, the tinge can not be controlled.
In addition, a stripe due to positional misalignment is another problem of the printer in which a joint head is mounted. When the heads are arranged in a joined manner, due to an assembly error of the heads, there occurs uneven density of the dots discharged from the joint portion of the heads, and density unevenness in a stripe shape occurs. In addition, such a stripe may be generated due to the discharge of ink from nozzles itself, and there are many cases where particularly an end portion of the head has discharge characteristics different from those of other portions due to the generation of crosstalk, an air current, or the like, and accordingly, discharge banding or discharge skipping may easily occur therein. Furthermore, in the case of a color that is represented by overlapping a plurality of colors such as a multi-order color, since the tinge is adjusted by using a method of arranging dots, therefore, even when the tinge is adjusted in each color, there may be a case the way colors are overlapped is changed so that the way of exposition of the colors can not be controlled, or there may be a case where, due to the difference in the way of penetrating, the tinge can not be controlled.
As means for solving such problems, there is an overlap processing technique. The technique has features in which end portions of heads configuring the joint head physically overlap each other, and the overlapping portion forms an image by forming dots by using nozzles of both heads in a shared manner. Thus, since the characteristics of the nozzles of the two heads adjacent to each other are mixed in dots formed by the overlapping portion, the problem of a stripe can be alleviated. For example, in Japanese Patent Application Laid-open No. 2004-50445, relating to the overlap processing technique, a technique is disclosed in which a nozzle from which dots are formed is determined based on a random number.
Particularly in a line printer, since there are many heads that configure the joint head, it is difficult to perform position adjustment in the joining portion of all the heads at high precision. In addition, since it is difficult to perform multi-pass printing in which an image is formed by a plurality of scanning operations in the line printer, the landing misalignment of the dots on a sheet due to assembly misalignment of the heads, discharge banding of ink, or the like may easily affect the image. Accordingly, in the printer in which such a joint head is mounted, this technique is used together with the above-described tone correcting technique for each head in many cases.
However, when the tone correction for each head and the overlap processing between adjacent heads are simply performed together, there may be still a case where a density (shading) stripe is generated in the overlapping portion. in other words, in a case where it is divided, for example, at the center of the overlapping portion and tone correction operations that are appropriate for the head a and the head β are respectively performed, although the overlap processing is performed in the overlapping portion, on the head α side and the head β side, discharged dots that are optimized for the other head side are present. Accordingly, the overlapping portion is formed to be thin or thick, and as a result, there is a case where a density stripe is generated in the overlapping portion.
In other words, as illustrated in FIG. 16, in a case where a discharged dot of the head α is large, and a discharged dot of the head β is small, when discharge data is changed by the center of the overlapping portion (joint portion), as illustrated in the dot pattern D3, the dot arrangement is sparse in a portion onto which only the head α discharges ink, and the dot arrangement is dense in a portion onto which only the head β discharges ink. In the joint therebetween, on the head α side, although small dots discharged by the head β are mixed, the dot arrangement may be a sparse arrangement, which is optimized for the head α, so as to have a low density, and, on the head β side, although large dots discharged by the head α are mixed, the dot arrangement may be a dense arrangement, which is optimized for the head β, so as to have a high density.
There is a need to solve such problems. There is a need to prevent the generation of a density stripe.